GRADE 12 SBA PRACTICAL 1: DNA EXTRACTION 17 FEBRUARY 2025

TIME: 60 MIN TOTAL = 30 MARKS

This task contains 7 pages including background information, procedure and

write-up

Practical Skills assessed: Following instructions, handling apparatus, making

observation, recording data, measuring, interpretation, designing an
investigation and identifying variables

THE EFFECT OF TEMPERATURE ON DNA EXTRACTION FROM BANANAS

INSTRUCTIONS:

1. Your educator will explain how the practical is to be done before the lesson.
2. You will go home and make sure that you understand what needs to be done. In other
words learn everything written in this hand-out. You will not be allowed to use the
handout in class.
3. You will perform the practical in class during the 1 hour lesson and your educator will
assess your work.
4. You will work in groups of six but your write-up must be done individually.

DNA Extraction – Background

All living cells have DNA. We will be
extracting DNA from bananas. The soft flesh
of a banana makes it easy to mash and
provides a ready source of DNA. This will
break down the cell walls.
The DNA molecule is an invisibly thin, very
long strand. Even though DNA is invisible to
the naked eye, no microscopes are needed!
The reason is that you release so many DNA
strands that they tangle together into a thick
cable, visible without magnification.

Detergent in the extraction buffer is used to

dissolve the cell membranes and nuclear membranes so that the DNA can be released. The
membranes are made of lipids (fat) and the detergent will cut through the membrane. The
enzymes in the soap are breaking down the lipid molecules of the cell and nuclear membranes,
releasing the contents of the cell, including the DNA.

Salt (NaCl) is present in the extraction buffer and it will enable nucleic acids to precipitate out of
an alcohol solution as it causes the DNA strands to clump together.

Now you have a mixture of smashed cell walls, dissolved membranes and dissolved DNA and
other cell parts. This mixture is filtered through filter paper. DNA is a very long molecule but it is
small enough to pass through the holes in the filter paper.

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 DNA is soluble in water but not in alcohol. Using ice-cold ethanol (or methylated spirits) and ice-
cold water increases the yield of DNA because the cold conditions causes the chemical reactions
to take place more slowly (by slowing down the activity of the enzymes) so this slows down the
rate at which DNA breaks up. Cold ethanol helps the DNA to precipitate more quickly. Thus a
layer of alcohol is carefully laid on top of the filtrate. Any DNA that contacts the alcohol will clump
together, pulling the rest of the DNA strand along behind it. Soon you should see white strands of
DNA bubbling their way up from the extract. The DNA is less dense than water or cell scum----so it
floats up into the alcohol layer, where you see it as a snotty, string-like substance, with small
bubbles formed on it.

The DNA may be collected by twirling a bamboo skewer or glass stirring rod in the solution. The
DNA will spool itself around the skewer and can be pulled out of the solution.
Precipitate: solid material that comes out of solution due to a chemical or physical change

MATERIALS
* 1 banana cut into 3 equal parts (Use a scale to ensure each piece is the same weight)
* 3 Ziplock sandwich bags
* To make DNA extracting solution:
5ml Salt
½ cup cold water / ½ cup water at room temperature / ½ cup warm water
* 3 x ½ teaspoon (2.5ml) Dish washing liquid
* 3 test tubes (label A – room temperature water, B- cold water, C – warm water)
* 3 filter paper, paper towel or wound swabs
* beaker that measures 125ml
* bamboo skewers or sosatie sticks
* Ice cold ethanol/methylated spirit (place test tube of ethanol in a beaker containing ice cubes)
* Thermometer
* Scale

Recipe for DNA Extracting solution:

Mix: 5ml (1 teaspoon) salt
½ cups (125ml) of water at three different temperatures (room, cold and hot)

PROCEDURE
1. Place ONE piece banana in a ziplock bag and
remove most of the air before you seal the
bag. Mash the banana in the bag into a
smooth puree by squishing it with your fingers.
Do not hit against the table and risk the bag
breaking.
2. Mix the salt and water to make the extraction
solution by making use of water at room
temperature / tap water.
Record the temperature of the water you use.

3. Open the bag and add the DNA extracting

solution which you have made up.
4. Gently mix the banana puree and extraction solution.
5. Add the ½ teaspoon dishwashing liquid to the bag and mix gentle. Do not shake to vigorously or
it will make a lot of bubbles and won’t filter effectively.
6. Set up a filtration system. Place a piece of filter paper or a coffee into your funnel.
7. Carefully pour the extract into the funnel. Allow the juice to filter through the towel into the
container below. Let it drip for 3-5 minutes.
8. The filter paper can be put inside the plastic bag and thrown away.

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9. Carefully transfer liquid from the beaker or cup into the test tube until the test tube
is about a third full.
10. Slowly add about 3 ml of ice cold alcohol to the test tube. The alcohol should be
added so that it trickles down the side of the tube before pooling on top of the
banana extract. You should end up with a yellow bottom layer and a clear top
layer.
11. Leave the solution for 2-3 min and watch carefully what happens in the clear
alcohol layer.
12. After 2-3 minutes, a skewer or stirring rod can be inserted into the tube and gently swirled
around. This will spool the DNA around the stick. The DNA can be pulled out of the tube. You
may touch the DNA but under no circumstances can you taste it.
13. Repeat the procedure but this time use cold water to make up the extraction solution
14. Repeat the procedure one last time and now use hot water to make up the extraction solution.
Note that you must record the temperature of the water used to make the buffer each time.

15. Compare the amount of DNA extracted in each of the 3 procedures.

The amount of DNA can be measured in arbitrary units…which are not
exact measurable units and can be any value based on the ratio of DNA
extracted and compared. You may use your cell phone to take photos
as evidence for your write up.
16. Show your educator the DNA which you have extracted.
17. Clean up.
[Link]

SUGGESTION: As you are working in a group of six, work in pairs and each pair will be
responsible for doing the experiment at a different temperature.

Assessment Criteria 0 1 2
Organisation – Procedure Procedure partly Procedure organised
worked through disorganised organised efficiently
procedure in orderly
manner
Efficient use of time Experiment not Experiment
allocation completed in time completed in time
Results / Final No DNA precipitated DNA precipitate can
product out be seen
Cleaned up after No Yes
practical

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 DNA EXTRACTION PRACTICAL PRESENTATION to be handed in by each learner

Name: __________________________________________ Mark: ___________ / 30

Group Members:
________________________________________

________________________________________

WRITE UP
1. State the aim of your investigation. (1)
To determine the effect of temperature on DNA extraction

2. Identify the following variables:

(a) Independent variable (1)

temperature

(b) Dependent variable (1)

DNA extraction

(c) Any TWO fixed variables (2)

Same type/amount of detergent

Same amount of alcohol

Same amount of water

Same type and amount of salt

Same amount/ mass of banana

Same banana species/type

Alcohol at the same temperature

Same amount of dishwashing liquid 

(ANY 2)

3. How could you make this investigation more reliable? (1)

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 Repeat the investigation at each water temperature

4. How can you improve the validity of this investigation? (2)

Same banana species/type

5. Write a conclusion based on your results. (2)

The colder the ethanol (or methylated spirits) and ice-cold water increases the yield of DNA
Extracted.

6. Why is it necessary to mash the bananas? (1)

This will break down the cell walls.

7. Explain why detergent is used. (1)

The enzymes in the soap are breaking down the lipid molecules of the cell and nuclear
membranes, releasing the contents of the cell, including the DNA.
8. What is the purpose of the salt? (1)

It will enable nucleic acids to precipitate out of an alcohol solution as causes the DNA
strands to clump together.

9. Explain why cold alcohol is used in this practical? (1)

The cold conditions causes the chemical reactions to take place more slowly (by slowing
down the activity of the enzymes) so this slows down the rate at which DNA breaks up./

Cold ethanol helps the DNA to precipitate more quickly.

10. What could you do to increase the amount of DNA precipitate that is formed in your
investigation? (Besides changing the temperature!) (1)

Use very cold alcohol/ add more salt

TOTAL: ________/15

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 PROCEDURE

Assessment Criteria 0 1 2
Organisation – Procedure Procedure partly Procedure organised
worked through disorganised organised efficiently
procedure in orderly
manner
Efficient use of time Experiment not Experiment
allocation completed in time completed in time
Results / Final No DNA precipitated DNA precipitate can
product out be seen
Cleaned up after No Yes
practica

TOTAL: __________ / 5

2.1 Grade 12 learners performed an experiment to show the amount and appearance of DNA found in
three different fruits based on mass and observations.
The amount of DNA for each fruit was calculated by subtracting the amount of alcohol and the
amount of juice from the mass of the filtered juice.

The alcohol was measured at 50 grams for a ¼ cup. The results of the calculations lead to the
answer of how much DNA is present in each of the fruits. An average of the results of the three trials
was established.
The following formula was used.
A–B–C=D
A = Mass of the filtered juice
B = Mass of alcohol
C = Mass of juice (without alcohol and DNA)
D = Mass of DNA

The amount of DNA present in each experiment and also the average of DNA present in the
different fruits.
Values for Values for kiwis Values for
strawberrie bananas (g)
s (g)
(g)
Trial 1 A-B-C=D A-B-C=D A-B-C=D

65- 50- 14= 1 75- 50- 24 =1 62- 50 - 10= 2

Trial 2
68- 50- 13= 5 78- 50- 25 =3 64 - 50- 12= 2
Trial 3
71- 50- 16= 5 73- 50- 21 =2 62 - 50- 11= 1
Average mass of 3.6  2 1.6
DNA (g)

2.1.1 Complete the table by filling in the missing values and calculate the Average mass of DNA
in each fruit (3)

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2.1.2 How was the reliability of the results ensured (1)

Three trials were done/ repeated three times and average mass of DNA calculated
Or
Measurement of DNA mass for each fruit was done three times and the average mass
calculated

2.1.2 Use the information in the table to draw a bar graph that shows the average mass of DNA
in each of the fruit. (6)
(10)

Average mass of DNA in three different fruit

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3.5
3
average mass (g)

2.5
2
1.5
1
0.5
0
Strawberrie Kiwis Banana
Type of fruit

RUBRIC FOR GRAPH MARKING

CRITERIA ELABORATION MARK
Caption (C) includes both independent Type of fruit)and 1
dependent variables ( amount of DNA) correctly
stated
Correct type of graph Bar graph drawn 1
drawn (T)
X and Y axes label (L) X and Y axes correctly labelled with units 1
X and Y axes scale (S) - Equal space and width of bars for X axes 1
and
- Correct scale for Y axes
Plotting of coordinates (P) - 1 to 2 co-ordinates plotted correctly. 1
- All 3 co-ordinates plotted correctly 2
I6)
NB if Line graph/ histogram drawn lose marks for Type of graph and Scale
Transposed axes
- Can get full credit if axes labels are also swapped and bars are horizontal
- If labels are not corresponding, then lose marks for labels and scale
- Check that the plotting is correct for the given labels

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